In recent years, remarkable progress has been made in terms of reducing the thickness and size of semiconductor chips. In particular, the semiconductor chips installed in IC cards such as memory cards and smart cards require a chip having a thickness of not more than 75 μm and a size of not more than 10 mm×10 mm. It is thought that as the demand for IC cards increases in the future, the requirement for small, thin chips will continue to grow.
Semiconductor chips are usually obtained by processing a semiconductor wafer to a prescribed thickness in a back grinding step or etching step or the like, and subsequently dividing the semiconductor wafer into individual chips in a dicing step. In the dicing step, a blade cutting method in which the semiconductor wafer is cut with a dicing blade is generally used. In the blade cutting method, minute defects (also referred to as “chipping”) are sometimes formed on the semiconductor chip as a result of the cutting resistance generated during the cutting process. This occurrence of chipping not only impairs the external appearance of the semiconductor chip, but depending on the degree of chipping, can sometimes cause damage to the circuit pattern on the semiconductor chip, and has recently been recognized as an important problem. In semiconductor chips that have been reduced in thickness and size, the permissible level of chipping is particularly stringent. It is expected that in the future, as further reductions are made in the thickness and size of semiconductor chips, the problem of chipping will become more critical.
The stealth dicing method is another method that can be used in the dicing step. The stealth dicing method is a method particularly suited for cutting ultra thin semiconductor wafers, and it is known that the stealth dicing method is able to suppress chipping. The stealth dicing method is performed in the following manner.
In one specific example of the stealth dicing method, first, a laser beam is irradiated onto the semiconductor wafer so that the focal point of the laser beam is located inside the interior of the semiconductor wafer, thereby forming a brittle modified section inside the semiconductor wafer as a result of multiphoton absorption. By moving the irradiation position of the laser beam along the lines which indicate the positions where the semiconductor wafer is to be divided into individual semiconductor chips (also referred to as “intended cutting lines”), modified sections (also referred to as “dicing lines”) can be formed along the intended cutting lines. Subsequently, a dicing tape is affixed to the rear surface of the semiconductor wafer (the surface on which no circuit is formed), and the dicing tape is then expanded. As a result, external stress is applied to the semiconductor wafer, and the semiconductor wafer cleaves along the intended cutting lines, thus dividing the wafer into individual semiconductor chips and widening the spaces between the semiconductor chips.
The step of expanding the dicing tape is usually called an expansion step. The expansion step is a step that is generally conducted regardless of whether the dicing step is performed using the blade dicing method or the stealth dicing method. When the dicing step is performed using the blade dicing method, the semiconductor wafer is divided into semiconductor chips using the dicing blade, and the expansion step is then performed to widen the spaces between the semiconductor chips. In this case, the expansion step is conducted mainly for the purpose of facilitating pickup of the semiconductor chips. However, when the dicing step is performed using the stealth dicing method, then as described above, the expansion step functions as a step for dividing the semiconductor wafer in which the dicing lines have been formed into individual semiconductor chips. Further, in addition, the expansion step also functions as a step for widening the spaces between the divided semiconductor chips. Examples of the methods and apparatus used in the expansion step are disclosed in Patent Literatures 1 to 3 and the like.